Multi-pass, arcuate bent waveguide, high power super luminescent diode

ABSTRACT

An optical device ( 300 ) including first and second facets ( 340, 350 ); an at least partially bent waveguide ( 320 ) formed on a substrate and including a portion perpendicular to the first facet; and a light amplification region ( 310 ) coupled to the bent waveguide. The light amplification region includes an expanding tapered portion and a contracting tapered portion which approaches the second facet.

RELATED APPLICATION

[0001] This application claims priority of U.S. patent applicationserial No. 60/185,133, entitled “DOUBLE-PASS HIGH POWER SUPERLUMINESCENTDIODE (SLD) AND OPTICAL AMPLIFIER WITH MODE STABILIZATION”, filed Feb.25, 2000, the entire disclosure of which is hereby incorporated byreference herein.

FIELD OF INVENTION

[0002] The present invention relates generally to optical devices, andparticularly to superluminescent diodes (SLD's) and lasers.

BACKGROUND OF INVENTION

[0003] There is currently a need for high power SLD's suitable for useas optical amplifiers. It is an object of the present invention toaddress this need. There is further a need for compact and reliableprojections systems. It is another object of the present invention toaddress this need as well. The invention also enables one to provide andenable external cavity lasers.

SUMMARY OF INVENTION

[0004] An optical device including: first and second facets; an at leastpartially bent waveguide formed on a substrate and including a portionperpendicular to the first facet; and, a light amplification regioncoupled to the bent waveguide, the light amplification region includingan expanding tapered portion and a contracting tapered portion whereinthe contracting tapered portion approaches the second facet.

BRIEF DESCRIPTION OF THE FIGURES

[0005]FIG. 1 illustrates a double-pass bent or arcuate waveguidesuperluminescent diode (SLD) utilized according to an aspect of thepresent invention;

[0006]FIG. 2 illustrates a mode propagating through the waveguide ofFIG. 1;

[0007]FIG. 3 illustrates a type of waveguide utilized according to yetanother aspect of the present invention;

[0008]FIG. 4 illustrates another type of waveguide utilized according toyet another aspect of the present invention;

[0009]FIG. 5 illustrates a up-conversion fiber laser according toanother aspect of the present invention;

[0010]FIG. 6 illustrates an up-conversion laser utilizing a PPLN crystalaccording to another aspect of the present invention;

[0011]FIG. 7 illustrates a display system according to an aspect of thepresent invention; and,

[0012]FIG. 8 illustrates modal reflectivity of an angled stripe SLD at1550 nm wavelength for several lateral index steps.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Referring now to the figures, like references there-throughoutdesignate like elements of the invention. A double-pass or bentwaveguide superluminescent diode (SLD) 10 utilized according to oneaspect of the present invention is shown in FIG. 1. The SLD 10 includesa ridge waveguide portion 20, having an effective index of refractionn_(e), along a substrate 30 having an index of refraction n_(c). Theeffective index is obtained from the active layer's bulk index n_(f) bysolving Maxwell's equation with the waveguide's boundary conditions. TheSLD 10 includes a first facet 40 having a coating of a prescribedreflection, which may be a highly reflective coating or ananti-reflective (AR) coating and a second facet 50 having an (AR)coating. The lateral index step for the ridge waveguide 20 has arefractive index difference Δn≦0.01 according to an aspect of thepresent invention. The ridge waveguide 20 width (w) is such as tomaintain a single transverse-mode, typically about 3 μm for operatingwavelength in the 1000 nm range. Referring now also to FIG. 8, it showsmodal reflectivity of an angled stripe SLD at 1550 nm wavelength forseveral lateral index steps. It should be noted that selecting a tiltangle of θ≈6° (FIG. 1) and a waveguide 20 width (w) (FIG. 1) of about 3microns, the mode reflectivity is advantageously relatively constant fora Δn ranging from approximately 0.003 to approximately 0.01. Thisadvantageously increases manufacturing tolerances for the waveguide 20.

[0014] Referring again to FIG. 1, the radius of curvature is given by${r > {\frac{24\pi^{2}}{\lambda_{z}^{2}}{\xi }^{3}}},$

[0015] where ${\lambda_{z} = \frac{\lambda}{n_{e}}},$

[0016] where n_(e) is the effective index and λ the wavelength in freespace, and$\xi = \frac{\lambda}{2{\pi \left( {N_{e}^{2} - N_{c}^{2}} \right)}^{1/2}}$

[0017] This expression can be simplified to${r > {\frac{0.33762\sqrt{Ne}}{\left( {\,^{\Delta}n} \right)^{3/2}}\lambda}},$

[0018] where

[0019] Δn=n_(e)−n_(e). The length s of the bent region is given by s=rØ,where Ø is expressed in radians. For a ridge waveguide structure with anangle of 6° (˜0.1 radian), the length of the bend region is 0.1r, and arobust angle design with negligible bend loss is one for which r=10 mmand s=1 mm. For a chip of length 1 mm, the whole bent waveguide wouldsimply be a circular arc, and the bend loss would be of the order of 1%or less.

[0020] This configuration was also described in U.S. patent applicationSer. No. 09/396,597, filed Sep. 15, 1999 and entitled“MULTIPLE-WAVELENGTH MODE-LOCKED LASER”, the entire disclosure of whichis herein incorporated by reference. Referring now also to FIG. 2, thereis shown mode propagation through the SLD 10.

[0021] A “diamond-like” shaped SLD is taught in U.S. patent applicationSer. No. 09/205,575, filed Dec. 4, 1998 and entitled “LIGHT EMITTINGSEMICONDUCTOR DEVICE”, also hereby incorporated by reference herein. The“diamond-like” SLD structure thereof is capable of high power operation.This is possible because the walls of the waveguide are non-parallel, soit does not support high order waveguide modes. This allows itsfabrication in a volume that is much larger than a conventional narrowstripe SLD, and hence gives it the capability for emitting high power ina single mode.

[0022] According to the present invention, the bent SLD configuration ofFIG. 1 and the aforementioned diamond-like structure are used to producea high-power single-mode SLD 300 as is shown in FIG. 3, for example. Thediamond structure 310, or alternatively a single taper active structure,is placed at an angle α, typically 5 to 10° with respect to the frontfacet, and connected to a section 312 of narrow single-mode waveguide bymeans of a bent or arcuate waveguide 320 that is designed analogously tothe SLD shown in FIG. 1. According to a preferred form of the presentinvention, the transition from the narrow waveguide portion to the bentwaveguide portion should be gradual and not abrupt. Likewise, it ispreferred that the transition from expanding to contacting tapering inthe diamond-like region be gradual and not abrupt or point-like. In thisconfiguration 300, the narrow single-mode waveguide 312 is perpendicularto the back facet 340, so that light reflected from the back facet 340is re-injected into the diamond section or taper 310. The radius ofcurvature (r) is chosen as prescribed earlier in order to preventundesirable radiation and hence loss from the curved or bent portion 320of the waveguide. The width of the narrow waveguide portion 312 near theback facet 340 is such that it preferably propagates only the lowestorder mode. This width (w) is preferably about 3 microns for typicalridge laser structures. This further serves to stabilize the propagatedmode in the tapers of the portion 310 and further ensure single modeoperation.

[0023] According to an aspect of the present invention, the back facet340 is coated with an interference filter that provides highreflectance, for example >95%. In some other aspects, such as somedesigns of external cavity lasers, it can also be anti-reflectioncoated. The front facet 350 is coated with an anti-reflection coating toincrease output power according to another aspect of the invention. Thewaveguide layer structure can take the form of any typical laser diodestructure including an active emission layer sandwiched between p and ncladding layers deposited epitaxially on a semiconductor substrate (GaAsfor wavelength below 1,100 nm, or InP for wavelengths 1,300 nm to 2,000nm, for example). Therefore, the waveguide structure can be fabricatedusing a conventional process of photolithography, etching,metallization, and facet coating. Upon application of an electriccurrent to the device, light is created by spontaneous emission, and asmall component of it propagates along the waveguide guide where itundergoes gain by stimulated emission and is output as AmplifiedSpontaneous Emission (ASE).

[0024] It should be understood that in a single-pass SLD device, theoutput light is the guided ASE component emanating from the back end ofthe structure and propagating with exponential gain toward the front endor output facet. According to another aspect of the present inventionthough, the output of the device 350 also includes light emanating fromthe front end 350, propagating toward the back facet 340, beingreflected from the back facet 340 and emerging from the front facet 350after two passes through the structure. As a result, the maximum outputpower in the double-pass structure 300 is advantageously proportional tothe square of the gain of the device, whereas in the single-pass deviceit is only proportional to the gain. Thus, the double-pass SLD 300 isadvantageously a more efficient gain medium than a single-pass device.

[0025] Still referring to FIG. 3, the double-pass device 300 thereofalso offers advantages in its use as a gain medium in external cavitylasers (ECL). In order to make a laser from the device, in the casewhere the back facet has a high-reflect coating, all that is needed isto provide a feedback partial reflection from the output facet 350,since high reflection is already provided at the back facet 340.

[0026] The laser can be made tunable by using a frequency-selectivefront feedback, such as provided by a grating. The gain-bandwidth of theSLD at 1550 nm is about 100 nm. This value would also be the tuningrange of the laser. In this configuration, the wavelength tuning occursat the output end. An alternate configuration of the ECL is one in whichboth front and back facets are anti-reflection coated and in which theexternal feedback element provides maximum reflection. In this case, theoutput is taken from the non-angled facet, separating the tuningfunction from the output function.

[0027] One convenient application of the configuration with thehigh-reflect back coating is in the generation of high powerup-conversion light. As will be discussed, using such a configurationone can readily generate high power blue (˜460 nm) or green (˜520 nm)visible light from a gain medium of the type described herein byemitting radiation in the 910 to 930 nm range or 1020-1040 nm range,respectively.

[0028] Still referring to FIG. 3, according to another aspect of thepresent invention the device 300 can be formed so as to have a length of2300 μm and height of 600 μm. The angle of the diamond structure ispreferably about 6°, while the angle θ₁ defining the angle of the uppertaper of the section 310 with respect to the output facet 350 is about5.3°, and the angle 02 defining the angle of the lower taper of thesection 310 with respect to the output facet 350 is about 6.7°. The tiltangle is between 5 to 7° for a weakly guided angled stripe SLD at 1550nm. The length of the tapered portion 310 is preferably about 2150 μm,with L₁≈L₂≈975 μm. The taper angle θ₃ is preferably about 2° or less, inorder to maintain adiabatic condition, a condition in which very littlelight is converted to high-order waveguide modes. The transition of theregion 310 from tapering in an expanding manner to a contracting mannerpreferably happens in a length L₃ which may be about 200 μm. The lengthof the waveguide portion 312 is about 125 μm, which makes L₅approximately 2300 μm. In any event, L1 is approximately 125 nm, L2 is975 nm, L3 is 200 nm, L4 is 975 mm and L5 is 25 nm. Thus, the total L isabout 2300 nm. Basically, the length of the arcuate or curved part 310is selected to minimize radiation at 1555 nm.

[0029] Referring now also to FIG. 4, according to another aspect of thepresent invention, the single-mode waveguide section 412 coupled to adiamond or taper 410 can also be applied without bending the waveguideof the structure 400 to make a high-power single-mode laser. In thiscase, both the front 450 and the back facets 440 are perpendicular tothe waveguide structure 430. Again, the back facet 440 is coated with ahigh-reflect filter and the front facet 450 is coated with ananti-reflection layer to provide reflection on the order of a fewpercent. Such a structure can lase without external feedback, due toreflection provided by the back facet 400 and front facet 450. However,it should be recognized that the structure 400 is less suitable forexternal cavity lasers than the angled structure shown in FIG. 3. Forexternal cavity operation, front facet reflection is preferably <0.001%.This is more easily achieved when the waveguide 430 is not perpendicularto the front facet 450. For example, the angled structure 300 of FIG. 3which exhibits a 6° facet angle can provide reflection on the order of0.001% to 0.0001%.

[0030] Still referring to FIG. 4, the portion 412 is preferably about100 to 150 μm in length while the portion 410 is preferably about 2150to 2200 μm in length. The portion 412 preferably has a width (w) ofabout 3 μm, while the region 410 has a maximum width (w₁) of about 38 μmand the width (w₂) of the waveguide at the output facet 450 is about 10μm. The angle β₁ at which the taper of the region 410 expands ispreferably about 1° while the angle β₂ at which the taper of the region410 contracts is about 0.7°.

[0031] In U.S. patent application Ser. No. 09/201,032, filed Nov. 30,1998 and entitled “ALL SOLID STATE HIGH POWER BROADBAND VISIBLE LIGHTSOURCE”, also herein incorporated by reference, there is described anup-conversion laser including a high-power diamond infrared gain mediumand a length of rare-earth-doped fluoride fiber inside a double cavityfor high conversion efficiency. A fiber laser system 500 is shown inFIG. 5.

[0032] Still referring to FIG. 5, this laser system 500 is a dual-cavitylaser consisting of an infrared (IR) laser 510 of a type discussedhereto and that is pumped by an electric current, and a visible laser520 that is pumped by the IR laser 510. The IR laser 510 includes adiamond-like SLD 512, a high-reflect mirror 511 on the left side of thediamond-like region 512, and another high-reflect mirror 513 on the farright in FIG. 5, on an opposite side of the visible laser 520 from theIR laser 510. IR light passes through the fiber 521 of the visible laser520 and is partially absorbed by the rare-earth ions thereof to producefluorescence at a corresponding wavelength in the visible spectrum. Thevisible laser 520 includes the up-conversion fiber 521, a high-reflectmirror 522 for the visible on the left side of the fiber 521, and apartial-reflect mirror 523 on the right side of the fiber 521. Themirrors 522, 523 are preferably interference mirrors and providereflection only at the intended wavelength. Thus, the visible reflectors522, 523 are transparent to the IR light, and vice-versa.

[0033] Referring now to FIG. 6, according to another aspect of thepresent invention the up-conversion fiber 521 and external reflectors522, 523 are replaced by a crystal 610 such as a periodically-poledlithium niobate (PPLN) crystal 610, which includes a high-reflectvisible light reflector 611 in the back facet 614 thereof, ahigh-reflect IR reflector 612 and a partial-reflect visible reflector613. The configuration 600 of FIG. 6 also includes high-reflectreflector 621 on the back facet of the bent diamond-like SLD 620.Alternatively, a single tapered SLD could be used. FIG. 6 show anembodiment of the double-pass SLD dual cavity laser with the PPLNconfigured as a frequency converter. According to an aspect of thepresent invention, frequency conversion is obtained by frequencydoubling due to the non-linearity of the PPLN material characteristics.For example, if the IR pump wavelength is in the 910-930 nm range, theprocess of frequency doubling generates blue light at 455 to 465 nm.Similarly, if the IR pump wavelength is at 1020 to 1040 nm, then thelaser will generate a 510 to 520 nm green light.

[0034] A dual-cavity laser system according to the present invention iswell suited for generating primary light sources for color projectionsystems. It is an advantageously compact, being a few centimeters indimensions for example, point source which exhibits a low beamdivergence, and is suitable for high efficiency projection systems, withhigh saturation colors and a long lifetime. According to another aspectof the present invention, such systems can produce visible output powerin the 1 to 10 watt range.

[0035] Referring now to FIG. 7, according to yet another aspect of thepresent invention the light sources for a projection system 799 includeone source of the type described in FIG. 5 or 6 with the appropriate IRwavelengths for the generation of blue and green, respectively 720, 730.While, according to another aspect of the present invention, a redprimary beam can be generated directly from the semiconductor at 630 to650 m using the structure 710 shown in FIG. 4, and discussed above, forexample. Outputs from the sources 710, 720, 730 are supplied to apolarization cube 740 which feeds a projector 750, for example.

[0036] Although the invention has been described and pictured in apreferred form with a certain degree of particularity, it is understoodthat the present disclosure of the preferred form, has been made only byway of example, and that numerous changes in the details of constructionand combination and arrangement of parts maybe made without departingfrom the spirit and scope of the invention as hereinafter claimed. It isintended that the patent shall cover by suitable expression in theappended claims, whatever features of patentable novelty exist in theinvention disclosed.

What is claimed is:
 1. An optical device comprising: first and secondfacets; an at least partially arcuate waveguide formed on a substrateand including a portion coextensive to said first facet; and, a lightamplification region coupled to said arcuate waveguide, said lightamplification region including an expanding tapered portion and acontracting tapered portion wherein said contracting tapered portionapproaches said second facet.
 2. The device of claim 1, wherein saidcontracting region approaches said second facet of said optical deviceat an angle of about six degrees to perpendicular.
 3. The device ofclaim 2, wherein said at least partially bent waveguide has a width ofabout three microns.
 4. The device of claim 3, wherein said at leastpartially bent waveguide and amplification region are formed on asubstrate including said first and second facets and an index ofrefraction difference between said at least partially bent waveguide andamplification region and said substrate ≦approximately 0.01.
 5. Thedevice of claim 1, wherein said waveguide is a ridge waveguide.
 6. Thedevice of claim 1, wherein at least a portion of said at least partiallybent waveguide has a radius of curvature sufficiently large to curtailradiation of a mode propagating through said waveguide due to thecurvature thereof.
 7. The device of claim 1, wherein said radius ofcurvature is on the order of 1 mm.
 8. The device of claim 1, furthercomprising a fiber optic coupled to said tapered portion of saidwaveguide.
 9. The device of claim 8, wherein said fiber is doped so asto perform upconversion of light passing through said second facet. 10.The device of claim 9, further comprising a first plurality of frequencyselective reflectors coupled to said fiber.
 11. The device of claim 10,further comprising a second plurality of frequency selective reflectors,each of said second plurality of reflectors coupled to one of said firstplurality of reflectors.
 12. The device of claim 11, wherein each ofsaid first plurality of reflectors is highly reflective for at least oneselect frequency and highly transmissive for at least one otherfrequency.
 13. The device of claim 12, wherein at least one of saidsecond plurality of reflectors is highly reflective for at least oneselect frequency and highly transmissive for at least one otherfrequency.
 14. The device of claim 13, wherein at least one other ofsaid second plurality of reflectors is only partially reflective forsaid at least one select frequency that said at least one of said secondplurality of reflectors is highly reflective.
 15. The device of claim 1further comprising a crystal coupled to said second facet for performingupconversion of light passing through said second facet.
 16. The deviceof claim 15, wherein said crystal is a periodically poled lithiumniobate crystal.
 17. The device of claim 15, further comprising a firstplurality of frequency selective reflectors coupled to said crystal. 18.The device of claim 17, further comprising a second plurality offrequency selective reflectors, each of said second plurality ofreflectors coupled to one of said first plurality of reflectors.
 19. Thedevice of claim 18, wherein each of said first plurality of reflectorsis highly reflective for at least one select frequency and highlytransmissive for at least one other frequency.
 20. The device of claim12, wherein at least one of said second plurality of reflectors ishighly reflective for at least one select frequency and highlytransmissive for at least one other frequency.
 21. The device of claim13, wherein at least one other of said second plurality of reflectors isonly partially reflective for said at least one select frequency thatsaid at least one of said second plurality of reflectors is highlyreflective.
 22. The device of claim 1, further comprising a highlyreflective coating on said first facet.
 23. The device of claim 1,further comprising an anti-reflective coating on said second facet. 24.The device of claim 1, wherein a transition from said expanding taperedportion to said contracting tapered portion is gradual.
 25. A weaklyguided angled stripe SLD comprising, a first ridge waveguide portionextending into a second arcuate waveguide portion between a first outputfacet and a second output facet with said arcuate waveguide having atilt angle of between 5 to 10° with respect to said first ridge portion.26. The SLD according to claim 25 wherein said tilt angle is closer to60.
 27. The SLD according to claim 25 wherein said arcuate waveguideportion is selected of a length to minimize radiation loss.